OK will try to explain it as concisely as I can. lets take the OEM wheels:
Stock GT-R Wheels
Front 20 x 9.5 45 offset
Rear 20 x 10.5 25 offset
The 20 is the diameter, the 9.5 and the 10.5 are the widths. Let's convert those to mm(tires are specced in mm).
9.5" = 241mm
10.5" = 266mm ( we will ignore offset for now).
so for a 20" x 9.5" wheel, you want a tire that is close(doesn't have to be exact) to 241mm. So a tire like a 285/35 will probably fit this wheel, but it is going to hang over a bit. a 275/40 will fit a bit better, and a 255 is probably the "correct" fit. However it's a little subjective, and there isn't much harm in going a little big. However a 305/30 tire is definitely going to be too wide to put on a 9.5" wheel. Sure you can mount it, but It can possibly leak and probably will wear funny.
for the 10.5" wheel(266mm) you are probably safe up to a 305. 315, I know some people do it, but personally I think that it stretching it a bit(no pun intended).
So like wise, a 20" diameter wheel with a 285/35 tire on it is 507 mm (height of wheel) + (385 * .35)(height of sidewall) = 606 mm = 23.5 inches in diameter.
So at ride height you can measure from the top of the tire to the inside of your wheel well-that's your clearance. You want to maintain probably 3+ inches of wheel well clearance so that you suspension has enough travel length.(there are books written on this idea so I won't go any further on that for now).
Now as your tire gets wider and wider and your wheel offsets get bigger and bigger to get that "aggressive" look, you start to bring your tire closer and closer to your fender.Once you have gone "too wide" you can still be saved by camber settings, fender rolling, all the tricks in the book, but at some point, the wheel/tire combo is too tall and too wide and it will rub the fender or bump stop on the wheel well before full suspension travel has completed.